Add event listeners for the various stages of making an HTTP request. The event listener receives detailed information about the request, and can modify or cancel the request.
Each event is fired at a particular stage of the request. The typical sequence of events is like this:
onErrorOccurred can be fired at any time during the request. Also note that sometimes the sequence of events may differ from this: for example, in Firefox, on an HSTS upgrade, the
onBeforeRedirect event will be triggered immediately after
All the events, except
onErrorOccurred, can take three arguments to
- the listener itself
filterobject, so you can only be notified for requests made to particular URLs or for particular types of resource
- an optional
extraInfoSpecobject. You can use this to pass additional event-specific instructions.
The listener function is passed a
details object containing information about the request. This includes a request ID, which is provided to enable an add-on to correlate events associated with a single request. It is unique within a browser session and the add-on's context. It stays the same throughout a request, even across redirections and authentication exchanges.
To use the webRequest API for a given host, an extension must have the "webRequest" API permission and the host permission for that host. To use the "blocking" feature the extension must also have the "webRequestBlocking" API permission.
To intercept resources loaded by a page (such as images, scripts, or stylesheets), the extension must have the host permission for the resource as well as for the main page requesting the resource. For example, if a page at "https://developer.mozilla.org" loads an image from "https://mdn.mozillademos.org", then an extension must have both host permissions if it is to intercept the image request.
On some of these events, you can modify the request. Specifically, you can:
- cancel the request in:
- redirect the request in:
- modify request headers in:
- modify response headers in:
- supply authentication credentials in:
To do this, you need to pass an option with the value "blocking" in the
extraInfoSpec argument to the event's
addListener(). This makes the listener synchronous. In the listener, you can then return a
BlockingResponse object, which indicates the modification you need to make: for example, the modified request header you want to send.
To modify the HTTP response bodies for a request, call
webRequest.filterResponseData, passing it the ID of the request. This returns a
webRequest.StreamFilter object that you can use to examine and modify the data as it is received by the browser.
An object of this type is returned by event listeners that have set
extraInfoSpecargument. By setting particular properties in
BlockingResponse, the listener can modify network requests.
- An array of HTTP headers. Each header is represented as an object with two properties:
- An object describing filters to apply to webRequest events.
- Represents a particular kind of resource fetched in a web request.
- An object that can be used to monitor and modify HTTP responses while they are being received.
- Contains data uploaded in a URL request.
- The maximum number of times that
can be called in a 10 minute period.
- This function can be used to ensure that event listeners are applied correctly when pages are in the browser's in-memory cache.
- Returns a
webRequest.StreamFilterobject for a given request.
- Fired when a request is about to be made, and before headers are available. This is a good place to listen if you want to cancel or redirect the request.
- Fired before sending any HTTP data, but after HTTP headers are available. This is a good place to listen if you want to modify HTTP request headers.
- Fired just before sending headers. If your add-on or some other add-on modified headers in
, you'll see the modified version here.
- Fired when the HTTP response headers associated with a request have been received. You can use this event to modify HTTP response headers.
- Fired when the server asks the client to provide authentication credentials. The listener can do nothing, cancel the request, or supply authentication credentials.
- Fired when the first byte of the response body is received. For HTTP requests, this means that the status line and response headers are available.
- Fired when a server-initiated redirect is about to occur.
- Fired when a request is completed.
- Fired when an error occurs.
|Chrome||Edge||Firefox||Firefox for Android||Opera|
|Yes||Yes||54 *||54 *||Yes|
|Yes *||Yes *||46 *||48 *||Yes *|
|Yes *||Yes *||45 *||48 *||Yes *|
|Yes *||Yes *||45 *||48 *||Yes *|
The "Chrome incompatibilities" section is included from https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Chrome_incompatibilities using the WebExtChromeCompat macro.
If you need to update this content, edit https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Chrome_incompatibilities, then shift-refresh this page to see your changes.
- In Firefox requests can be redirected only if their original URL uses the
- In Firefox, events are not fired for system requests (for example, extension upgrades or searchbar suggestions). From Firefox 57 onwards, Firefox makes an exception for extensions that need to intercept
webRequest.onAuthRequiredfor proxy authorization. See the documentation for
- In Firefox, if an extension wants to redirect a public (e.g. HTTPS) URL to an extension page, the extension's manifest.json file must contain a web_accessible_resources key that lists the URL for the extension page. Note that any website may then link or redirect to that url, and extensions should treat any input (POST data, for examples) as if it came from an untrusted source, just as a normal web page should.
Microsoft Edge compatibility data is supplied by Microsoft Corporation and is included here under the Creative Commons Attribution 3.0 United States License.
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